Brett,
Yes, I do know about the re-partitioning of strain during deformation
(or let’s call it a model). I agree with you that growth of porphs may
be structurally controlled in any one particular site (that’s one of
many possibilities, anyway). In fact, what I’m saying is that the
overall volume of a specific mineral in a rock sample is primarily
controlled by P-T-X parameters, and if growth stops at one site, for
whatever reasons, growth must continue somewhere else in this sample.
For that reason, it had always been emphasized that
porphyroblast-matrix relations cannot be properly judged from single
thin sections.
However, Tim is saying that porphs grow generally early during
deformation. Now, you tell me what happens if a rock has evolved
beyond a differentiated crenulation stage. No more mineral growth,
until the infamous orthogonal event comes in? I have no problems with
multiple foliations forming during an orogeny (that’s my attempt at
diplomacy!), and I even published a paper with Tim about that (oops,
maybe I shouldn’t have mentioned that). But I have seen enough late-
and post-deformational minerals which also makes sense in each case.
I can’t see the reason why minerals should stop growing along a
prograde or retrograde P-T path if deformation has ceased. Well,
perhaps deformation never really ceases if T is high enough, but I
am not talking about some odd little bend in a mica grain. If you
look for that you’ll always find it.
What you mentioned about the Robertson River Metamorphics sounds very
interesting indeed. I thought about this type of work before, and
I am still looking for an appropriate area. A relatively simple
deformation history would be an advantage.
Cheers, Juergen
(Still waing for Tim's delivery of a broadside.... Tim, don't tell me
you are working on weekends but not at night!?)
J. Reinhardt
Dept. of Geology
University of Natal
Durban, 4041
South Africa
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
|